1. Field of the Invention
The present invention relates to a semiconductor device having an interlayer insulation film the required properties of which are liable to deteriorate by oxygen plasma treatment, and a process for producing the same. In particular, the present invention relates to a semiconductor device making it possible to restore deteriorated properties, and a process for producing the same.
2. Description of the Related Art
The demand for high-speed processing of signals in large scale integrated circuits (LSIS) has been increasing year by year. The speed of processing signals in LSIs is mainly decided by the operation speed of their transistors themselves and the delay time of signal-transmission in wiring. The operation speed of transistors, which has greatly influenced the speed of processing signals in the prior art, has been improved by scaling down the size of the transistors.
In LSIs having a designed size of less than 0.25 .mu.m, however, a decline in the signal-processing speed based on the delay in signal-transmission in wiring is becoming remarkable. Such an influence is high in LSI devices having a multilayer wiring structure wherein the number of wiring layers is over 4.
Recently, therefore, as a method for improving the delay in signal-transmission in wiring, there has been investigated a method of using, instead of an interlayer insulation film of conventional silicon oxide films, a hydrogen silsesquioxane (HSQ) film or the like, which has a smaller dielectric constant. The HSQ film is a resin film having a chemical structure wherein a part of Si--O bonds of a silicon oxide film is replaced by Si--H bonds. The film is applied onto a substrate and then heated and sintered so as to be used as an interlayer insulation film. Since almost all parts of the HSQ film are composed of Si--O bonds in the same way as in conventional silicon oxide films, the HSQ film has a low dielectric constant and heat-resistance, up to about 500 .degree.C.
However, when the HSQ film is used as an interlayer insulation film, there remains a problem that the HSQ film deteriorates in the step of exfoliating photoresist used for forming various patterns in usual lithographic technique and etching technique.
Generally, in the step of exfoliating photoresist, a treatment with oxygen plasma is conducted and subsequently remnants of the photoresist, which are not exfoliated, and etching remnants are removed. For this purpose, treatment with a wet exfoliating solution containing monoethanolamine or the like is conducted. When the HSQ film is exposed to oxygen plasma, Si--H bonds therein are broken and Si--OH bonds are generated so that the film comes to contain water. When the HSQ film is treated with the wet exfoliating solution, Si--H bonds are broken and Si--OH bonds are generated in the same way as in the treatment with oxygen plasma. That is, in these exfoliating steps, the HSQ film comes to contain a large amount of water. As a result, its dielectric constant unfavorably rises. If the HSQ film comes to contain a large amount of water, there arise a problem of leakage between vias. In the step of embedding in via holes by CVD or sputtering, the embedding in the via holes becomes insufficient by degassing.
The following will describe a process for producing a semiconductor device in the prior art. FIG. 1 is a cross section illustrating the semiconductor device producing process in the prior art.
An underlying layer 52 is first formed on a silicon substrate 51. The underlying layer 52 includes underlying layer elements such as transistors. Next, a barrier metal layer 53 is selectively formed on the underlying layer 52. Thereafter, a first metal wiring layer 54 is formed on the barrier metal layer 53. A reflection preventive layer 55 is formed on the first metal wiring layer 54. Next, a first silicon oxide film 57 is formed on the entire surface by plasma CVD. Subsequently, an HSQ film 58 is applied on the first silicon oxide film 57 by a coating machine. The resultant is subjected to provisional sintering on a hot plate, followed by sintering in a sintering furnace.
At this time, in order to prevent dissociation of Si--H bonds, nitrogen or the like is generally introduced into the space around the hot plate or in the sintering furnace so that the HSQ film does not react with oxygen or water. Next, a second silicon oxide film 59 is formed on the HSQ film 58 by plasma CVD or the like. Thereafter, a patterned photoresist is used to etch the second silicon oxide film 59 and the HSQ film 58 on the reflection preventive layer 55. In this way, via holes are made. Next, the photoresist is exfoliated by a treatment with oxygen plasma. The resultant is further subjected to an exfoliating treatment with an alkali-wet solution in order to remove etching remnants and the like.
As described above, at this time Si--H bonds in the regions of the HSQ film 58 which are exposed to the via holes and subjected to the oxygen plasma are changed into Si--OH bonds by the oxygen plasma treatment and the exfoliating treatment with the wet solution. Therefore, deteriorated portions 58b having an increased dielectric constant are produced in these regions. These deteriorated portions 58b cause poisoned vias.
Moreover, there is proposed a method of forming an HSQ film and subsequently subjecting the resultant to a plasma treatment with an inert gas, such as nitrogen or argon, from its surface in order to improve the strength of the HSQ film (Japanese Patent Application Laid-Open No. 8-111458).
According to this producing process in the prior art, disclosed in the above-mentioned publication, the strength of the HSQ film is improved. Thus, cracks are not easily produced even if external stress is applied to the HSQ film from a metal layer formed as an underlying layer of the HSQ film. Even by this method in the prior art, however, it is impossible to suppress the rise in the dielectric constant of the HSQ film.